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So the pair and their colleagues began hunting for more giant viruses in water sediments, where other big viruses have been found due to the abundance of amoeba prey.
Sure enough, they found two: Pandoravirus salinus, from the mouth of Chile's Tunquen River, and Pandoravirus dulcis from a freshwater pond near Melbourne, Australia—both of which parasitize amoebas.
"Finding such a new type of virus that is so different happens once every 50 years—it's a major discovery," said the team, whose study appears today in the journal Science.
Why haven't scientists found pandoraviruses before?
There are several reasons, but a simple one is that many scientists still assume viruses are small.
"When people look into cells and when they see things that don't have the right dimension or don't have regular assets or geometries, they don't think of viruses—they think its some kind of bacteria," Claverie and Abergel said.
When the scientists then try to cultivate these supposed bacteria in the laboratory and fail, it doesn't surprise them, since up to 60 percent of bacteria in the oceans can't be grown in the lab.
The study authors also note that Pandoravirus may had already been found 13 years ago—but scientists just didn't know what it was. When the team screened scientific literature on parasites that eat a type of amoeba called Acanthamoeba, they found mention of Pandoravirus-like particles.